Foldable LED light recycling cavity

ABSTRACT

LEDs are mounted onto a flat, thermally conductive, substrate, which is folded to form a light recycling cavity. A planar substrate is first coated with a metal layer, which is patterned to electrically connect the LEDs and to form bonding pads for wirebonds to connect the LEDs to external circuitry. The LEDs are mounted on the substrate. The substrate is then scribed on the backside to form the folds. The LED dies are then attached onto the metal islands (pads) defined on the substrate and wirebonds are used to connect the top side of the LED to adjacent patterned metal islands (pads) on the substrate. The substrate is then folded into a light recycling cavity where the LEDs are facing the inside of the cavity.

REFERENCE TO PRIOR APPLICATION

This application is a continuation of prior U.S. patent application Ser.No. 12/154,318, filed on May 21, 2008 now U.S. Pat. No. 8,029,165 andclaims the benefit of U.S. Provisional Patent Application Ser. No.60/931,094, filed on May 21, 2007, both which is herein incorporated byreference.

TECHNICAL FIELD

The present invention is related to the fields of semiconductorprocessing and light emitting diodes (LEDs). The invention is a methodfor assembling and manufacturing light recycling cavities. The lightrecycling cavities have a foldable substrate assembly for the LEDs,which attains high thermal conductivity for the LEDs and precisealignment of the LEDs when the substrate is folded to form the lightrecycling cavity.

BACKGROUND OF THE INVENTION

In U.S. Pat. Nos. 6,869,206, 6,960,872, 7,025,464, 7,040,774, and7,048,385, there is shown an enhanced light source, which can be formedby placing LEDs in a light recycling cavity. These light recyclingcavities can enhance the brightness of the LEDs which are used to formthem, thereby achieving gains in brightness greater than 2

Forming these light recycling cavities requires precision placement ofthe LEDs and reflecting surfaces inside the cavity and also a means todissipate the heat generated by the LEDs to exterior heatsinks. Forexample, in U.S. Pending Patent Application Publication NumberUS-2006-0092639-A1, “High Brightness Light Emitting Diode Light Source”,to William R. Livesay et al., these LED light recycling cavities can beintegrated with heatsinks.

However, not shown is a simple, inexpensive, and mass producible meansof forming these light recycling cavities. In these light recyclingcavities, unlike most conventional mounting methods, the LEDs are notplaced in a planar arrangement. The individual LEDs or an array of LEDsare aligned orthogonal to the other LEDs or LED arrays inside thecavity.

This orthogonal alignment is quite different than most electroniccomponent fabrication processes, which utilize planar fabricationmethods and planar substrates or circuit boards. A substrate and itsplanar arrangement are in 2 dimensions. The light recycling cavity andits orthogonal alignment is in 3 dimensions. This orthogonal alignmentmakes it difficult or impossible to use high speed and mass productionprocesses like surface mount, chip on board, etc. as conventionallypracticed with 2 dimensional substrates to form these light recyclingcavities in 3 dimensions. For example, a light recycling cavity may beformed by mounting LEDs on a thermally conductive substrate (aluminumnitride) and forming an array of LEDs thereon utilizing prior arttechniques. Multiple arrays (similarly formed) are then individuallymounted onto heatsinks, which are then brought into alignment to form alight recycling cavity.

However, precise alignment of these arrays to each other and within therecycling light cavity is required. This fabrication process can betedious and difficult, requiring that each array have intimate thermalconnection to an exterior heatsink. This requires attaching eachsubstrate to a separate heatsink and then joining all of the arrays andheatsinks together to form a cavity while maintaining alignment andprecise positioning of the LED arrays to each other within the cavity.Due to the three dimensional nature of the finished cavity, it isdifficult to use high speed manufacturing means such as pick and placemachines to mass produce these assemblies.

Therefore, there is a need for an improved method of forming these lightrecycling cavities that simplifies the assembly of the light recyclingcavity and maintains alignment of the LEDs within the light recyclingcavity. In U.S. Pat. No. 5,997,708, Craig discloses a foldable substrateutilizing a multilayer assembly. However, this assembly is complex andrequires bonding other materials to the substrate to provide a suitablehinge to effect the fold. This requires extra process and alignmentsteps.

In the prior art, electronic packages that have incorporated foldableelements have utilized flex circuits formed by a metal (for theelectrical interconnect) deposited onto a pliable substrate usuallyplastic (e.g. polyimide, Mylar, etc.) to form the foldable material orhinge.

However, there are several drawbacks to these prior art techniquesparticularly in an assembly requiring high alignment accuracy and highthermal conductivity. Most polymers have very low thermal conductivityand low maximum service temperatures. If the polymer is used only forthe hinge material, it must be somehow attached to the target substrate.This attachment requires an adhesive which must also have a maximumworking temperature sufficiently high to allow die attachment processesto be performed. This die attachment process also adds an additionalprocess step to form the foldable assembly that is not easily performedin a planar process.

The hinge, if so attached, requires that it have excellent adhesion tothe substrate (to the edge of the substrate at the folding joint) sothat alignment of the LEDs of the light recycling cavity is maintainedwhen the substrate is folded. Most polymer materials require a minimumthickness for mechanical robustness. However, the minimum bend radius isat least the thickness of the material. For example, if a 0.025 mm(0.001″) thick polyimide layer is used as the hinge material, thisrequires at least at least a 0.025 to 0.050 mm or greater bend radius.This variable bend radius imparts an uncertainty to the position of thesubstrate parts when folded. A requirement for a recycling light cavityis to have very few or small gaps between the LEDs within the cavity.This uncertainty in the position of the LEDs requires larger spacing(between LED arrays so they will not mechanically interfere) and lowerscavity efficiency.

Therefore, there is a need for a highly accurate means of forming theselight recycling cavities without increasing process steps.

SUMMARY OF THE INVENTION

The invention is a method for assembling and manufacturing lightrecycling cavities having a foldable substrate assembly for the LEDs.The light recycling cavity attains high thermal conductivity for theLEDs and precise alignment of the LEDs when the substrate is folded toform the light recycling cavity. Prior art techniques incorporateflexible substrates, extra processing steps, or dedicated materials toform hinges. The present invention discloses a novel foldable substrateassembly while not requiring any additional process steps than thatrequired for making conventional planar LED arrays.

Specifically, the invention comprises a foldable substrate wherein LEDsare mounted onto a flat, thermally conductive substrate (e.g. aluminumnitride). The substrate (aluminum nitride) is electrically isolating butthermally conductive.

This invention discloses a foldable substrate assembly that utilizessimple processing steps similar to those processing steps required toassemble a conventional planar array and requires no extra processingsteps to form or add a hinge. A planar aluminum nitride substrate isfirst coated with a metal layer 1 to 15 micrometers thick. As is donewith planar arrays, this metal layer (Au, Ag, Cu, Ni, etc.) is patternedto electrically connect the LEDs in series or parallel arrangement andto form bonding pads for wirebonds to connect the LEDs or externalcircuitry. The metal layer is thick enough (preferably 3 μm to 12 μm) tocarry the current (0.1 to 10 amps) required for driving high brightnessLEDs.

Prior to LED die attach, the aluminum nitride substrates are partiallyscribed, cut or sawed on the backside (of the substrate) to allowbreaking of the substrate to form walls for a cavity. The sawing orcutting of the substrate is done in such a way as to not cut the metalpatterned (gold) layer on the topside.

Next, the LEDs are mounted (die attached) via solder or conductiveepoxy. This mounting allows the LEDs to be placed on a substrate viahigh speed pick and place equipment as the substrate assembly is in aplanar orientation for LED mounting and adding interconnects(wire-bonding, etc.). The metal layer is pre-deposited (plating,sputtering, evaporation, etc.) on the foldable substrate and ispatterned prior to the placement of the LEDs and, therefore, ensuringthat the LEDs are aligned to the substrate and relative to themselves.This pre-deposition eliminates the alignment step required to form lightrecycling cavities by using discrete substrates to form the sides,bottoms, etc.

In one of the preferred methods of this invention, an aluminum nitridesubstrate is coated with a metal layer on both sides. One preferredmetal layer is gold in a thickness layer of 3 to 15 microns. The metallayer is patterned to form the interconnects to the LEDs, for example, atop and bottom electrode running in close proximity to where the LEDwill be mounted. The metal layer may also be patterned to providealignment marks or solder dams for accurate placement of the LEDs. Thesepatterned metal islands or electrodes may be configured to provideinterconnects to each of the LEDs in either a series or parallelconfiguration. The patterning of the metal can be via conventionalphotolithography using either a subtractive etching process or anadditive plating process or alternatively, the patterned metal islandinterconnects may be formed by cutting the gold with a laser ablationprocess.

The substrate is then scribed or cut on the backside of the substrate(opposite side from the patterned metal). The scribing can be done viaeither a diamond saw, laser, or other such means. The scribing is doneto cut through the bottom side metal layer, the substrate (e.g. aluminumnitride), but not through the top side patterned metal layer.Alternatively, the substrate is only partially cut and later cracked toform the folds. In this way, the substrate will remain rigid in a planarconfiguration for die attach and wirebonding.

The LED dies are then attached with a eutectic solder onto the metalislands (pads) defined on the substrate. Wirebonds (gold, silver,aluminum, etc.) are used to connect the top side (cathode) of the LED toadjacent patterned metal islands (pads) on the substrate. Because allthese processes are performed on a planar substrate assembly, high speedconventional processing methods may be used (e.g. pick and place, etc.).

The 2 dimensional substrate is then folded into a 3 dimensional lightrecycling cavity where the LEDs are facing the inside of the cavity. Thethin metal layer (gold, silver, nickel, platinum, etc.) which providesthe electrical connection also acts as the hinge that allows thesubstrate to be folded into a three-sided trough, four-sided cavity, ora five-sided box. Since the patterning of the metal and scribing is donein a precise manner, and the LEDs are placed in a precise manner whenthe substrate is folded, it forms a perfectly aligned cavity and can bepositioned into a simple slotted heatsink and secured via solder orthermally conductive adhesive.

Synergistically, this invention solves the difficult problem ofelectrically connecting LEDs or arrays of LEDs that are oriented at 90°and 180° to each other in forming a light recycling cavity. This problemis solved because the hinge material is also the interconnect to theadjoining sides of the cavity, thereby seamlessly crossing from one side(LED array) to another. The thin metal interconnect layer will be robustenough to act as a hinge to a rigid substrate packed with LEDs. Thecavity only has to be folded once and then is secured in a mechanicallystable holder of a heat spreader.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a top plan view of a three-section patterned metal coatedsubstrate of the present invention.

FIG. 1B is a bottom view of the three-section metal coated substratewith cut grooves of the present invention.

FIG. 1C is a cross-sectional view of the channels cut in the undersideof the substrate of the present invention.

FIG. 1D is a top plan view of the three-section patterned metalsubstrate with LEDs and interconnects shown of the present invention.

FIG 1E is a cross-sectional view of a three-section patterned metalsubstrate folded into a three-sided cavity of the present invention.

FIG. 1F is a cross-sectional view of a three sided cavity withreflective end caps forming a five-sided cavity of the presentinvention.

FIG. 1G is a cross-sectional view of a slotted heat spreader of thepresent invention.

FIG. 1H is a perspective view of the completed three LED faced lightrecycling cavity of the present invention.

FIG. 2A is a top plan view of a four-section patterned metal coatedsubstrate of the present invention.

FIG. 2B is a bottom view of a four-section metal coated substrate withcut grooves of the present invention.

FIG. 2C is a top plan view of a four-section metal coated substrate withLEDs and interconnects shown of the present invention.

FIG. 2D is a cross-sectional view showing the detail of one foldedcorner of the four-section (sided) cavity of the present invention.

FIG. 2E is a simple perspective view of the folded four-sided cavity ofthe present invention.

FIG. 3A is a plan view of an unfolded five-sided LED light recyclingcavity of the present invention.

FIG. 3B is a top view of a folded five-sided cavity showing theinterconnect detail of the present invention.

FIG. 3C is a simple perspective view of the folded five-sided cavitywith interconnect tabs.

FIG. 4A is a top plan view showing an alternate method to form end capsfor a three-sided cavity of the present invention.

FIG. 4B is a cross-sectional view showing extra tabs forming end capswhen folded of the present invention.

FIG. 4C is a cross-sectional view of the extra flaps forming a taperedmirrored end caps of the present invention.

FIG. 5A is a top view of scribed unfolded substrate with LEDs attachedof the present invention.

FIG. 5B is a perspective view showing the flex circuit overlay andsubstrate prior to attachment of the present invention.

FIG. 5C is a perspective view showing the flex circuit attached to thethree section substrate of the present invention.

FIG. 5D is a perspective view of the folded cavity on the flex circuitof the present invention.

FIG. 5E is a perspective view showing the folded cavity with end caps ofthe present invention.

FIG. 5F is a perspective view showing the copper clad PCB with milledslot of the present invention.

FIG. 5G is a perspective view showing the completed folded recyclinglight cavity in the PCB heatsink of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will be betterunderstood by those skilled in the art by reference to the above listedfigures. The preferred embodiments of this invention illustrated in thefigures are not intended to be exhaustive or to limit the invention tothe precise form disclosed. The figures are chosen to describe or tobest explain the principles of the invention and its applicable andpractical use to thereby enable others skilled in the art to bestutilize the invention. The above listed figures are not drawn to scale.In particular, the thickness dimension of the substrate, LEDs, andmetallization layer, which acts as both an electrical interconnect andhinge, is expanded to better illustrate the various layers of theembodiments.

FIG. 1A shows a top plan view of a three-section light recycling cavitysubstrate assembly 100 prior to folding into a cavity. This particularrecycling light cavity will have three sides with a single LED or aplanar array of LEDs on each of the three sides. This view shows thesubstrate before the LEDs are mounted. The substrate can be fabricatedfrom aluminum nitride, copper tungsten, alumina, etc. Alignment marks110, 115, 120 are patterned into the metal layer for accurate placementof the LEDs or LED arrays to the substrate. In this figure, thesubstrate is shown with a patterned metal layer which forms theinterconnects between the LEDs (LEDs are not mounted or shown in thisview). This metal layer is preferably made from a high electricalconductivity metal (e.g. gold, copper, silver, etc.).

The metal layer is first deposited onto the substrate by sputtering,evaporation, plasma jet, electroplating, or other means. The metal layeris then patterned into isolated islands that form bond pads to connectto the base of the LED, isolated islands for wirebonding to the topcontacts of the LED and isolated islands to connect to external power.The metal layer may be patterned using a laser to etch or ablate themetal or, alternatively, the metal may be patterned using aphotolithography process wherein the gold is coated with photoresist,the photoresist exposed through a mask, developed, and the gold etchedthrough the photoresist mask. The metal is patterned to forminterconnects to the LEDs and between LEDs on adjacent sides. Forexample, the patterned metal forms an electrical island 132 which willmake contact with the bottom contact of an LED or LEDs placed thereon.

Island 134 provides a wirebonding pad for wirebonds that make contact tothe top surface contact (cathode) of the LED mounted on island 132.Island 134 is also the bottom (anode) contact for the center LED arrayaligned to alignment marks 120. Island 134 extends across the foldablejoint indicated by the dotted line 137. Island 136 provides the wirebondpad connection to the top contact (cathode) of the center LED array (onisland 134) and the bottom anode connection to the right-side LED array.Finally, island 138 provides the wirebond pad for the top contact to theLED or LED array mounted on island 136 and the contact for externalpower.

Optionally, a metal coating is also deposited on the backside of thesubstrate. The metal coating can be gold, silver, copper, etc. Thisbackside metal coating can be used to permit attachment of the completedassembly to a heat spreader or heatsink with eutectic solder. Thisbackside metal is otherwise unpatterned.

After patterning of the top metal layer, the substrate is scribed,sawed, or cut with a laser, saw, diamond scribe, etc. on the backside ofthe substrate (the side opposite of the patterned metal).

FIG. 1B shows the substrate with cut channels on the backside of thesubstrate. These cut channels correspond to the fold lines 137, 139shown in FIG. 1A.

FIG. 1C shows a cross-section of these cut channels wherein the channels142 extend through the optional backside metal 141 and through thesubstrate 100 or partially through the substrate, but not through thepatterned metal layer 144 on the opposite side of the substrate. In thisfigure, the thickness of the substrate and metal layer are not to scale.Preferable substrate thickness is 0.05 mm to 0.350 mm. Preferablethickness for the bottom metal layer 141 is 2 μm to 4 μm. Preferablethickness for the top patterned metal layer is 3 μm to 15 μm and morepreferably 5 μm to 10 μm. For this cutting process, a narrow kerf isdesirable, preferably smaller than 100 μm and more preferably less than25 μm, and even more preferably less than 15 μm. A narrow kerf will helpmaintain the precise alignment of the sides and LEDs when the cavity isfolded.

Next, the LEDs are mounted onto the patterned metal islands on thesubstrate. This is preferably done using a eutectic metal alloysoldering process, such as 80% gold /20% tin. This gold/tin eutectic istypically applied to the LEDs prior to this die-attach step or,optionally, the substrate can be coated with a solder eutectic. Thisgold/tin eutectic is typically 3 to 6 microns thick. Gold/tin eutecticsrequire heating to temperatures in excess of 300° C. to get a reliablevoid-free joint. Accordingly, all materials that are used on thesubstrate up to this point must be stable and not oxidized or melt atthese high temperatures. This prevents the use of most polymer flexcircuit materials.

Polymer materials used in flex circuits are flexible but haveintrinsically low thermal conductivity. Most substrates that exhibithigh thermal conductivity like aluminum nitride, aluminum, coppertungsten, etc. are rigid and brittle and cannot be folded withoutcracking. Therefore, to fabricate a foldable assembly with rigidsubstrates requires assembling separate substrates and joining them witha flexible material. However, that fabrication requires extra processsteps and requires alignment of the individual substrates if the finalassembly is to form a precisely aligned structure.

FIG. 1D shows the LEDs mounted by soldering onto the patternedmetallized substrate. After the LEDs are attached eutectically, the topcontacts (LED anode) are made via wirebonds 152, 154, and 162. Thisparticular configuration shows the wirebonds 152, 154, and 162 connectedvia a series arrangement wherein a positive voltage is applied to metalisland area 132 which extends underneath and contacts the bottom (anode)of the LED 156. The top contact (cathode) of LED 156 is made via thewirebonds 152 to metal island 134, which extends across the fold line137 and makes contact to the bottom (anode) of LED 158 centered on thebottom (center) of the three-sided recycling light cavity. The top sidecathode contact of LED 158 is connected via wirebonds 154 to metalisland 136 which extends across the fold line 139 and makes contact tothe bottom (anode) of LED 160. The top side contact (cathode) of LED 160is connected via the wirebonds 162 to metal island 138. This metalisland is connected to the negative side of a suitable power supplyproviding enough voltage and current to drive these LEDs in series. Thisconnection can be by a wire (not shown) soldered to tab 145 soldered tothe metal island. Tab 147 soldered to island 132 is similarly connectedvia a wire to the positive side of the power supply.

All of these steps in this assembly process to this point are performedon a planar substrate and, therefore, can utilize conventional highspeed pick and place, die attach, wirebond and other such assemblyequipment. The metal islands 132, 134, 136, 138 extend across the foldlines 137, 139, however, they are all isolated from each other by thelines 133 and 135 where the metal has been removed by laser orphotolithography as previously mentioned. However, metal islands 134 and136 extend across the fold lines to provide a continuous electrical pathfrom one section (side) to the other section (side). Therefore, thismetal layer provides a metal interconnect between LEDs and/or LED arraysinside the light recycling cavity and also acts as a mechanical hingeallowing the left and right side of the cavity to be folded vertically.This metal interconnect and hinge is shown in FIG. 1E where the twosides 170 and 172 have been folded vertically so the two side arrays ofLEDs 156 and 160 are now facing each other with the bottom LED 158(three sides have LEDs) or LED arrays facing up vertically between thesesides. This thin metal layer is robust enough to act as a hinge 163, 164and hold the sides together to form a cavity. Optionally, toelectrically isolate the metal hinge from the backside and helpmechanically support the hinge, a fillet of cement or adhesive such assilicon rubber, Resbond 989F, etc. may be applied to the joint area 165,167.

Shown in FIG. 1F is a perspective view of the folded cavity 170. Tocomplete this three-sided light recycling cavity, two highly reflectingend caps 174, 176 fabricated from expanded Teflon and/or highlyreflective mirror materials are inserted in the two open ends of thecavity. These end caps form the remaining sides of a five-sided lightrecycling cavity and also provide mechanical support to the structure,relieving stress on the thin metal hinges. The reflectivity of theinterior surfaces 178, 180 of these end caps is higher than 95%preferably higher than 98%. This highly reflective end caps, coupledwith high reflectivity LEDs, assure an enhancement in brightness for theLEDs inside the cavity. See, for example, U.S. patent application Ser.Nos. 6,869,206 and 6,960,872. After this process step, the lightrecycling cavity is fully formed and now only has to be placed into aheatsink.

Shown in FIG. 1G is a slotted copper plate 182, which acts as a heatspreader or heatsink. The inside dimensions of the slot 184 match theoutside dimensions of the folded cavity 170 shown in FIG. 1F. Forexample, the width of the slot may be 2 to 2.4 mm wide and the height ofthe slot may be 2 to 2.4 mm high for a light recycling cavity containing12 (1 mm×1 mm) LEDs, consisting of 3 arrays of four LEDs (2×2 array withone array per side. The through holes 186 in the slotted copper plateallow the heat spreader 182 to be mounted to a suitable heatsink.Several examples of heatsinks are given in pending U.S. patentapplication Ser. No. 60/811,310, “Light Emitting Diode Light Source withHeatsink” by William R. Livesay et al. The cavity is inserted into theslot 184 and can be secured by soldering eutectically, utilizing theoptional metal coating on the backside of the substrate, to makeintimate contact with the three aluminum nitride substrates, as shown inFIG. 1H. A eutectic solder is used that has a lower melting temperaturethan the one used to mount the LEDs onto the aluminum nitride substrate.For example, Sn96.5/Ag3.5 (221° C.) or Sn/Bi (138° C.). This solderingis done to ensure that the LEDs do not move or lose their alignment toeach other in this final fabrication step.

FIG. 1H shows a perspective view of the finished assembly 190 showingthe folded cavity 170 with end caps 174, 176. Also shown are two metaltabs 145 and 147 formed from gold, copper, etc. that have been solderedto metal islands 132 and 138 (as shown FIG. 1D). These tabs can be usedto connect power to the LEDs as previously described.

This sequence of process steps shows a simple, reliable, and repeatablemethod for assembling light recycling cavities. This unique approachmaintains precise alignment of the LEDs within the light recyclingcavity and only requires one additional process step over a process thatmight be used to form or mount and electrically connect arrays of LEDson planar substrates. The one additional process step is a simple one,that of scribing, cutting, or sawing the aluminum nitride substrate.This can be done accurately with a diode pumped 266 nm laser using twowatts in a 5 μm spot size which will result in a kerf width ofapproximately 12 μm.

As discussed in the prior art references cited in the “Background”section, light recycling cavities can attain higher intrinsic gain orlight amplification by increasing the number of LEDs inside the lightrecycling cavity. Using a process similar to the one depicted in FIG. 1which shows a three-sided cavity, a four-sided cavity may be constructedusing a few additional steps.

FIG. 2A shows the substrate 202 with a patterned metal electrode for afour-sided cavity. The metal is deposited and patterned as previouslydescribed, however, instead of three sections for mounting LED arrays,there are four sections 208 and three fold lines 210, 212, 214 which,when folded, become the four sides of a four-sided cavity. As can beseen in FIG. 2A, the patterned gold forms metal islands 224, 217, 218,219, 226 to interconnect the LEDs to be mounted such that each LED arrayor LED or each side is connected in series. The output of thisfour-sided cavity would be as shown by the arrow 220. The metal islandsare shortened on this output side to move the LEDs next to the outputaperture of the completed cavity.

Referring to FIG. 2B, the substrate 202 is cut on the backside (oppositeside from the patterned metal) creating three channels 205, 211, 213that allow the assembly to be folded. The LEDs are mounted andwirebonded using the process described previously and depicted in FIG.1.

A different interconnect arrangement is shown in FIG. 2C to facilitatelining up the LEDs to the edge of the substrate, which will become theoutput edge of the cavity. The connection to an external power supply(positive terminal) to power this cavity is made through the metalisland 224, which contacts the bottom (anode) of LED 232.

The top contact (cathode) of LED 232 makes contact to electrode 217through wirebonds 242. The metal island 217 extends across the fold line210 and under LED 234 to make contact to the anode of LED 234. The topcontact (cathode) of LED 234 makes contact to the metal island 218through the wirebonds 244. Metal island 218 extends across the fold line212 and under LED 236 to make contact to the anode of LED 236. The topcontact (cathode) of LED 236 makes contact to metal island (electrode)219 through wirebonds 246. The wirebonds 246 make contact to metalisland 219 to the left side of the fold line 214. This is also done withwirebonds 242 to metal island 217. This orientation keeps the wirebondsfrom getting bent when the cavity is folded. The top contact (cathode)of LED 238 makes contact to metal island 226 through the wirebonds 248.The negative polarity terminal of the power supply is applied bysoldering a wire or metal tab to metal island 226. The placement ofwirebonds 242 and 246 are novel in that when the cavity is folded, aportion of the wirebonds are hidden from the active part of the cavityas shown in FIG. 2D.

Shown in FIG. 2D is a cross-section (for illustrative purposes, not toscale) of the two sides of the cavity 252, 254 showing how theinterconnect is made between LED 236 and LED 238. LED 236 is attached tosubstrate 252 through metal island 217. The top contact (cathode) of LED236 is connected to the patterned metal island 219, which is attached tosubstrate 252 and to substrate 254. This top contact connection is madevia wirebond 246. As shown in this figure, metal island 219 runs fromjust outside LED 236 and runs across and underneath LED 238 makingcontact to the anode of LED 238. A portion of the wirebond 246 is hiddenfrom the rest of the cavity in area 260 formed when the cavity isfolded. This area is defined by the thickness of the LEDs, which canvary from 50 microns to 300-400 microns. For current high brightnessLEDs, the LED thickness is approximately 100 microns. One advantage ofthis interconnect method is that the gold wire which is typically usedto make the wirebonds is not highly reflective for blue and greenwavelengths and, therefore, obscuring a portion of the gold wire fromthe light recycling cavity enhances the overall efficiency of thecavity.

The folded version of this four-sided cavity is shown in a simplifiedperspective view in FIG. 2E. To complete this cavity, a bottom, highlyreflective end cap 292 similar to ones previously described (see FIG.1F) is placed at the bottom of the light recycling cavity.

A five-sided cavity can be constructed using the process steps previousdescribed with one additional section and fold to form the bottom of thecavity. A planar substrate 300 is shown in FIG. 3A for fabricating afive-sided cavity. Also shown are LEDs mounted and wirebond connectionsto show the interconnect method for this cavity. The process forfabricating the substrate, metallizing, patterning the metal, scribingthe substrate, performing die attach and wirebonding have all beendescribed previously. For this five-sided cavity, the interconnectbetween the LEDs on each side is in a series arrangement.

It is instructive in describing the interconnect method of thisfive-sided cavity to refer to FIGS. 3A and 3B in the description thatfollows. For clarity, single LEDs are used to cover each side, however,multiple LED arrays may be used for each side. FIG. 3A shows thefive-sided cavity before it is folded while still in a planararrangement. FIG. 3B shows the five-sided cavity after it is folded witha top view shown looking down inside the cavity from the open end.Referring to FIG. 3A, a positive polarity connection is made to tab 310,which is connected or optionally part of metal island 312, which makescontact to the bottom (anode) of LED 314.

The top contact (cathode) of LED 314 is connected via wirebonds 316 tometal island 318 which extends across fold line 320 and underneath LED322 to make contact to the bottom anode of LED 322. The top contact ofLED 322 is connected to metal island 324 via the wirebonds 326. Themetal island 324 extends across the fold line 328 and underneath LED 330to make contact to the bottom anode of LED 330. The top contact of LED330 is connected via wirebonds 332 to metal island 334 which runsunderneath LED 336 making contact to the bottom anode of LED 336. Thetop contact of LED 336 makes contact to the metal island 338 viawirebonds 340. In this case, this connection is at right angles to theother in-line connections of the four sides of the cavity, as the tab342 is hinged off the metal island 338 to form the bottom of the cavity.

Metal island 338 contacts the bottom anode of LED 344. Wirebonds 346connect the top contact of LED 344 to the metal island 348. Metal island348 extends out on tab 350 to enable electrical contact to be made tothe inside of the folded five-sided cavity. This arrangement is shown inFIG. 3B and FIG. 3C which depicts the five-sided cavity folded with tab350 and tab 310. A wire 352 may be soldered to the metal island 348 ontab 350. The positive polarity connection is made on wire 354 which issoldered to metal island 312 and sticking out on tab 310. In thisconfiguration, the electrical connections to the chain of LEDs is madeat 310 and 350.

These are extra tabs, which are sticking out beyond the flap 320, thatforms the bottom of the cavity and the sides. In this arrangement,five-sided cavity wirebonds are made to an isolated island, whichconnects the next die in series. The wirebonds made from the top of thefirst die make connection to the bottom of the second die so that whenfolded the wirebonds are partially hidden by the cavity 364, 366, etc.formed by the edges of the die (as shown in FIG. 3B). This connectionarrangement allows the die to be placed with their edges right at theedge 293 of the output aperture of the cavity, at least for the fourside arrays. Practicing this invention, one skilled in the art can mixthese elements to configure any number of combinations of lightrecycling cavities.

Another embodiment of this invention incorporates additional flaps,which provide an easily constructed three-sided cavity with end caps asshown in FIG. 4A and 4B. FIG. 4A shows a three-section 402, 404, 406substrate similar to FIG. 1A. However, the center section 404, whichwill form the bottom of the cavity, has two extra flaps 410 which isconnected to 412, and 414 which is connected to 416. Metal islands arepatterned and deposited as previously described, however, an extraprocess step is added wherein the bottom of the cavity has a mirrorcoating of dielectric, protected silver, etc. deposited via sputter,evaporation, and/or electroplating.

Shown in FIG. 4B is a cross-sectional view showing just end flaps 410,412, 414, and 416 folded. The end flaps 410, 412, 414, 416 when foldedwill form a mirrored inside surface to both inside ends of the cavity.The mirrored surfaces 422 and 424, which were on the bottom of thesubstrate, after folding, will form the inside mirrored ends replacingthe end caps shown in FIG. 1F. This makes forming the end cap simple andself-aligning. The folding can be done to form either a verticalmirrored surface 422 and 424 in FIG. 4B, or a tapered mirrored surface432, 434, as shown in FIG. 4C.

The heretofore examples of the embodiments of the invention were shownusing single LEDs per side or section to more clearly illustrate theinvention. However, for some light source applications multiple LEDs maybe required for each side of the cavity to achieve higher output. Bysuitable patterning of the metal islands shown in FIGS. 1, 2, and 3, oneskilled in the art can attach and connect multiple LEDs or arrays ofLEDs in series or in parallel arrangement in place of the single LEDsshown in these drawings. This will permit large recycling light cavitiesto be made for very high light output applications.

Multiple LEDs and a different method of implementing the folding cavityincluding connecting elements and insertion into a copper clad PCBTheatsink is shown in FIGS. 5A through 5G. In the previous examples, thehinge forming the foldable cavity is located on the substrate orsubmount, typically aluminum nitride, on which the LEDs are bonded.However, to provide convenient external connection to the LEDs andinterconnected bond pads on the substrate, another means of constructingthe foldable cavity is preferred. In this method, a foldable flexiblecircuit is mounted to the substrate. The flexible circuit is made bycoating an insulating plastic film, such as polyimide (Kapton), Mylar,etc. with metal such as copper, gold, etc.. The metal is etched in apattern to form interconnects to the appropriate locations. In thisembodiment, the flexible circuit contains the hinge, not the substrate.

Referring to FIG. 5A, the LEDs 502 are first bonded by soldering orepoxying to the metal pads 506 of the substrate 504. The metal pads canbe gold, copper, etc. There is no metallization forming a hinge acrossthe scribe lines 508 on the substrate, as shown in the previousembodiments.

A flex circuit 510 in FIG. 5B is attached via solder or epoxy to thesubstrate 506. The bottom of the flex circuit has metal pads 512 thatmake contact and are soldered to corresponding metal pads 504 on thesubstrate 506. In this view, for clarity, the printed circuit on the topof the flex circuit dielectric is not shown.

In FIG. 5C, the flex circuit 510 is shown bonded to the substrate 506.In this view, the printed circuit on the top of the flex circuitdielectric is shown. The topside patterned copper circuit 520 of theflex circuit shown is connected to the LED bond pads 522 by wire bonds524. The bond pads 522 (cathode connection) on top of the LEDsinterconnect to the top metal circuit 520 and by vias 526 through holesin the dielectric of the flex circuit to the adjacent LED anodeconnections through the metal pads 504 on the substrate 506 which areconnected to the bottom of the LEDs. The flex circuit is scribed, forexample by a laser, from the bottom side for the two folds that willform the cavity. These scribe lines are located so that they superimposeover the scribe lines in the substrate. The top layer metal conductor onthe flex circuit spans across the scribe lines in the flex circuit. Thethickness of this flex circuit is selected such that it matches theheight and thickness of the LEDs that are mounted on the substrate. Thisthickness matching allows the cavity to be folded with a hinge that islocated at the top emitting face of the LED. Any lateral displacement ofadjacent LED edges is minimized when the cavity is folded.

Once the flex circuit is attached to the substrate and, prior tofolding, wire bond interconnects are made between the LED cathode bondpads and the flex circuit top metal interconnect. The flex circuit cannow be folded forming the three sides of the recycling light cavity, asshown in FIG. 5D. The two sides 530 of the cavity fold on the hinges 532formed by the top metal on the flex circuit. The top metal is preferablya ductile metal, such as gold, copper, etc., and preferably between 5 to25 microns thick. When the fold is made, the substrates 506, typicallyaluminum nitride, beryllium oxide, alumina, copper, etc., break apart ontheir previously scribed lines and the bottom and sides fold on thehinge 532 formed by the top metal on the flex circuit.

To complete the cavity, two reflective end caps 540 are placed so as toclose up the two open ends of the cavity as shown in FIG. 5E. Tocomplete external connections, the flexible circuit extends 542 aboveand outward from the folded cavity and is scribed along the top edge ofthe cavity. This scribe is done from the topside 544 of the flexcircuit. The bottom conductor on the flex circuit, therefore, forms ahinge and also carries the electrical connection to the external circuitand connector.

A thick copper 550 clad printed circuit board 551 in FIG. 5F with amilled slot 552 acts as a receptacle for the folded cavity and also actsas a heatsink and heat spreader for the light source. The patternedcircuit 554 on the top of the PCB and pads 556 interconnect to matchingpads on the bottom of the flex circuit.

Referring to FIG. 5G, an external connector 560 is soldered to the cladPCB to provide external connections, such as power connections, to theLEDs in the light recycling cavity. The recycling light cavity 562 isinserted into the milled slot 552 in the copper clad PCB and soldered orepoxied into place. The final and completed connections are made byattaching the folded (at 90°) extended portion 564 of the flex circuitonto the matching interconnect pads 556 on the top of the copper clad PCboard 570. This bond can be made either via solder, conductive epoxy,etc.

As can be seen from this invention, multiple combinations of theseelements can be constructed using the basic premise of the invention.The method is elegant and simple and provides for high speed, highvolume manufacturing on planar substrates and arrays yet forms a threedimensional, fully functional light recycling cavity.

While the invention has been described in conjunction with specificembodiments and examples, it is evident to those skilled in the art thatmany alternatives, modifications and variations will be apparent inlight of the foregoing description. Accordingly, the invention isintended to embrace all such alternatives, modifications and variationsas fall within the spirit and scope of the appended claims.

1. A foldable light recycling cavity comprising a planar substratehaving a first side and an opposite second side, said planar substratehaving multiple segments; a flex circuit having a first side and anopposite second side, said opposite second side of said flex circuitbeing bonded to said first side of said planar substrate, said flexcircuit being metal; at least one cut channel in said second side ofsaid planar substrate between said multiple segments, each of said atleast one cut channels forming at least one fold line on said first sideof said planar substrate; at least two light emitting diodes mounted onsaid first side of said flex circuit; and wherein folding said planarsubstrate along said at least one fold line forms a light recyclingcavity, said flex circuit acting as a hinge during folding said planarsubstrate, and said flex circuit on said multiple segments of saidplanar substrate forms the walls of said light recycling cavity.
 2. Thefoldable light recycling cavity of claim 1 wherein said at least twolight emitting diodes are each mounted on a bond pad on said first sideof said flex circuit.
 3. The foldable light recycling cavity of claim 1wherein said flex circuit is copper.
 4. The foldable light recyclingcavity of claim 1 wherein said flex circuit has a ductile metal on firstside of said flex circuit.
 5. The foldable light recycling cavity ofclaim 4 wherein said ductile metal is gold.
 6. The foldable lightrecycling cavity of claim 4 wherein said planar substrate is aluminumnitride, beryllium oxide, or alumina.
 7. The foldable light cavity ofclaim 1 wherein flex circuit metal pads on said opposite second side ofsaid flex circuit are soldered to planar substrate metal pads on saidfirst side of said planar substrate.
 8. The foldable light cavity ofclaim 1 wherein at least one scribe line in said opposite second side ofsaid flex circuit superimpose on said at least one cut channel in saidsecond side of said planar substrate.
 9. The foldable light cavity ofclaim 1 wherein the thickness of the flex circuit matches the thicknessof the height and thickness of the at least two light emitting diodes.10. The foldable light recycling cavity of claim 1 further comprising afirst highly reflective end cap mounted in a first open end of saidlight recycling cavity and a second highly reflective end cap mounted ina second open end of said light recycling cavity.
 11. The foldable lightrecycling cavity of claim 1 further comprising four segments in saidplanar substrate and three fold lines in said planar substrate whichform a four sided light recycling cavity with at least one lightemitting diode on each side of said light recycling cavity.
 12. Thefoldable light recycling cavity of claim 1 further comprising fivesegments in said planar substrate and four fold lines in said planarsubstrate which form a five sided light recycling cavity with at leastone light emitting diode on each side of said light recycling cavity.13. The foldable light recycling cavity of claim 1 wherein said flexcircuit extends outside said light recycling cavity for externalconnections.
 14. A method of forming a light recycling cavity comprisingthe steps of: bonding the second side of a flex circuit on a first sideof a planar substrate, said planar substrate having multiple segments;cutting at least one channel in a second side of said planar substratebetween said multiple segments, said second side of said planarsubstrate being opposite said first side of said planar substrate, eachof said at least one cut channels forming at least one fold line on saidfirst side of said planar substrate; mounting at least two lightemitting diodes on said first side of said flex circuit, said first sideof said flex circuit being opposite said second side of said flexcircuit; and folding said planar substrate along said at least one foldline to form a light recycling cavity, said flex circuit acting as ahinge during folding and said flex circuit on said multiple segments ofsaid planar substrate forming walls of said light recycling cavity. 15.The method of forming a light recycling cavity of claim 14 wherein threefold lines in said planar substrate and four segments in said planarsubstrate form a four sided light recycling cavity with at least onelight emitting diode on each side of said light recycling cavity. 16.The method of forming a light recycling cavity of claim 14 wherein fourfold lines in said planar substrate and five segments in said planarsubstrate form a five sided light recycling cavity with at least onelight emitting diode on each side of said light recycling cavity. 17.The method of forming a light recycling cavity of claim 14 furthercomprising scribing a line in said opposite second side of said flexcircuit superimposed on said at least one cut channel in said secondside of said planar substrate.
 18. The method of forming a lightrecycling cavity of claim 14 further comprising mounting a first highlyreflective end cap in a first open end of said light recycling cavityand mounting a second highly reflective end cap in a second open end ofsaid light recycling cavity.